There are a dozens of species of flies that may lay their eggs or larvae on a dead body. Each of these species will develop at a different rate depending on environmental conditions; especially temperature. These species are primarily found within three families; Blowflies, House flies, and Flesh flies. They can all arrive on a body within minutes of death and begin laying eggs in the natural orifices or wound sites.
The Blowflies (Family:Calliphoridae) are probably the most recognizable and familiar to the average person. They are the metallic blue, green, yellow, and other colored flies you may see around trash cans and latrines. Species of this family can be found above 11,000 feet in elevation down to sea-level and are probably the most commonly encountered on dead bodies in terms of numbers and diversity. Interestingly, the term “blow” fly gained prominence in several works from William Shakespeare.
The House flies (Family:Muscidae) are more commonly found in urban areas with high population densities. They typically arrive after the blowflies but this is not always the case. They are similar in appearance to Flesh flies in that they have a grey thorax with longitudinal black stripes but they are typically smaller in size (sometimes half) than the Flesh flies. Like the blowflies they will generally lay their eggs in the natural orifices of the body. Some species are even predacious and will feed on their larvae will feed on other larvae at the body.
The Flesh flies (Family:Sarcophagidae) can be found all over the world but especially in tropical and warm-weather environments. They have a grey colored thorax with several black stripes running the length of the body segment. The abdomen also has a black and grey checkered appearance. The biggest biological difference with this fly is the manner in which it deposits its larvae. While the Blowflies and House Flies lay eggs, the Flesh flies lay larvae (the eggs develop and hatch in the female abdomen). These flies have been reported to have a preference for rural areas of low human population densities but I have found them at rural crime scenes as well.
In your story it’s important to recognize some of these differences in appearance. If you describe a blue-bottle fly it is a Blowfly, not a Flesh fly. Likewise if you talk about the robust grey and black fly it is not a blowfly. Adding a little detail to your descriptions can help build a picture in the mind’s eye but making sure that picture is accurate will add strength to your credibility with knowledgeable readers.